This invention relates to an imaging system and, more specifically, to lithography.
Lithographic printing is a well known and established art. In general, the process involves printing from a flat plate depending upon the difference in properties between image and non-image areas for printability. In conventional lighography, the non-image area is hydrophilic while the image is hydrophobic. A fountain solution is applied to the plate surface which wets all portions of the surface not covered by the hydrophobic image. This solution keeps the plate moist and prevents it from scumming up during the printing phase of the process. An oil-based printing ink is applied to the image surface, depositing the lithographic ink on the image area, the hydrophilic non-image area repelling the ink. The ink image may then be transferred directly to a paper sheet or other receptive surface, but generally it is transferred to a rubber offset blanket which, in turn, transfers the print to the final paper copy. Hence, for each print made during a run, the lithographic plate is dampened with an aqueous fountain solution, inked with a lithographic printing ink and printed via an offset blanket onto the final receptive copy sheet.
It has been known that lithographic plates can be made electrophotographically by utilizing conventionally developed electrophotographic plates as lithographic printing masters. In these systems, usually a zinc-oxide type of plate is charged by conventional means and exposed to the image to be reproduced with the resulting electrostatic latent image developed with conventional electrostatic toner. The toner is generally hydrophobic in nature, as is the undeveloped background area of a conventional binder-type electrophotographic plate. In order that the developed plate be useful as a lithographic master, a differential must be established between the toner image and the background of the plate. Since both are hydrophobic in nature, it is necessary to treat the background of the electrophotographic plate by the use of a conversion solution so as to render the background surface hydrophilic in nature. After the alteration of the non-image, background area, a nonaqueous, oil-based ink can be used whereby the toner will accept the ink and the now hydrophilic background areas will repel the ink.
While these systems have been found useful for lithographic purposes, there are inherent disadvantages in their use. For example, in the preparation of a printing master, it is necessary, in order to produce final printed images of lithographic quality, that the developed electrophotographic image be of extremely high-quality copy with sharp images of high-image density and minimal background so that the master produced can withstand the vigors of the lithographic process over extended periods of usage, a lithographic system inherently being a high-volume printing process. In the heretofore used developer systems in preparing electrophotographic printing plates to be utilized in lithographic printing processes, the images formed have been found to be less than adequate to produce the results required in lithography. Poor quality images have led to deficient masters which produce relatively short periods of usage time. In addition, high background and poor image density have contributed to the deficiencies.
It is, therefore, an object of this invention to provide a lithographic printing system which will overcome the above and other disadvantages.
It is a further object of the invention to provide a novel method for the preparation of a lithographic printing master.
Another object of the present invention is to provide an imaging system utilizing a novel lithographic master prepared from an electrophotographic plate.
Still a further object of the present invention is to provide a novel lithographic printing plate utilizing electrophotographic principles.
Yet, still a further object of the present invention is to provide a lithographic printing plate prepared by an electrophotographic system wherein the master produced is of a high-quality image with low background and high-image density and sharpness.
The foregoing objects and others are accomplished in accordance with the present invention, generally speaking, by providing a lithographic printing plate prepared electrophotographically wherein a single component developer composition, hereinafter referred to as a toner, for developing electrostatic latent images, is utilized comprising a resinous component having a sharp melt point and low melt viscosity essential for pressure fusing the exhibiting good cold-flow characteristics, inclusive of good wetting properties under pressure. For purposes of the present invention, the specific polyamide resin composition was determined to be highly suitable for use as a single component toner resinous component due to its sharp melting point characteristics and low melt viscosity. Included as a component of the polyamide toner composition is a magnetic oxide material, so that the final composition comprises 35-50 percent resin composition to 65-50 percent magnetic oxide. Below 35 percent resin, the adhesiveness of the toner will begin to deteriorate. In a preferred embodiment of the present invention, the particle size of the developer will range from a 10 micron absolute minimum to a 40 micron maximum resulting in less background, lower light exposure settings, better powder flow characteristics, less toner dust in the image development area and less tendency for the particles to spray over defined image edges, particularly around large solid areas.
The resulting polyamide-magnetic oxide toner composition possesses excellent cold-flow characteristics under pressure to exhibit unexpected pressure fixing characteristics. Since the subject resins have good cold-flow characteristics, they inherently possess the capability to desirably wet-out the magnetic oxide particles in the formulation. This wetting-out characteristic and relatively good cold-flow property of the toner is also attributed to the presence of the specific amount of magnetic oxide since the magnetic oxide has good dispersing characteristics and plays an important role in the cold-flow mechanism. The developer toner further preferably includes a highly conductive carbon pigment to regulate the resistivity of the resulting toner particle.
It has been determined in the course of the present invention that lithographic printing masters may be fabricated electrophotographically so as to produce a master having an extremely sharp image with minimal background utilizing a single component developer composition or toner comprising a pigmented polyamide resin and a magnetic oxide component to develop the electrostatic latent image. The instant developer composition exhibits the necessary characteristics which permit the toner to be used in an electrophotographic imaging process.
The utilization of the polyamide resin of the present invention will permit the formulation of a single component toner with a magnetic oxide content ranging from 50 to 65 percent by weight. The pressure fusible toners of the present invention are especially suitable for fabricating lithographic masters herein defined which are subsequently used for duplicating. The polyamide resins used in the process of preparing the printing master of the present invention provide for the required sharp melting point and low melt viscosity and corresponding necessary cold-flow characteristics, when used in combination with the magnetic oxide additive in the percentages prescribed, to provide an effective lithographic printing master.